The present invention relates to a magneto-optical storage medium and magneto-optical overwrite system.
Since high density-reading/writing is possible by utilizing a perpendicular magnetic film as a magnetic layer, many systems utilizing several kinds of magneto-optical storage media have recently been proposed for magneto-optical reading/writing systems in which data are written in a magnetic layer of a magnetic storage medium as a change of magnetization corresponding to the data, i.e. the magnetic layer has magnetization characteristics subject to heat applied thereto, and the data thus written in the magnetic layer is read by utilizing an optical effect such as the Faraday effect or the Kerr effect.
An optical disk memory employing a disk-format magneto-optical medium is interchangeable with respect to a reading/writing apparatus. Much effort is made to research and development of rewritable large capacity file memories and some are already being marketed. However, at present, a rewriting system for such file memories is not adequate, even though some memory systems are claimed rewritable, they are often incapable of direct overwriting.
The memory, accordingly, has such a disadvantage that it is not versatile, compared with, so called, a hard disk apparatus (for example, a Winchester hard disk apparatus) which has widely been used but the disk is built into the apparatus thus not interchangeable.
In order to overcome the disadvantage, several magneto-optical overwrite systems have been proposed, the following two systems being the examples of such.
(1) A magneto-optical overwrite system in which a revolving magneto-optical storage medium is irradiated by a tiny spot of light with constant intensity and the writing bias magnetic field is modulated in magnitude by the data signals to be written.
(2) A magneto-optical overwrite system which employs a magneto-optical storage medium having a first magnetic layer and a second magnetic layer, each of them being composed of an amorphous alloy consisting of a heavy rare earth metal and a transition metal, and the two layers being so laminated that there exists a magnetic exchange interaction therebetween, further the coercive force Hc.sub.1 of the first magnetic layer and the coercive force Hc.sub.2 of the second magnetic layer have the relationship Hc.sub.1 &gt;Hc.sub.2 at a room temperature. Furthermore, the Curie-temperature Tc.sub.1 of the first magnetic layer and the Curie-temperature Tc.sub.2 of the second magnetic layer have the relationship Tc.sub.1 &lt;Tc.sub.2. There is also a means for irradiating the magneto-optical storage medium with a laser beam the intensity of which is modulated into the first intensity level which causes the first magnetic layer being heated to a temperature higher than the Curie-temperature Tc.sub.1 and also the second magnetic layer being heated to a temperature higher than the Curie-temperature Tc.sub.2, and into the second intensity level which causes the first magnetic layer being heated to a temperature higher than the Curie-temperature Tc.sub.1 and also the second magnetic layer being heated to a temperature lower than the Curie-temperature Tc.sub.2. The system also comprises a means for focusing the laser beam with respect to said magneto-optical storage medium, an external magnetic field applying means for initializing the second magnetic layer by magnetizing only the second magnetic layer into a predetermined polarity in the thickness direction thereof before the magneto-optical storage medium is irradiated with the laser beam. In a cooling process subsequent to the demagnetization of the both first and second magnetic layers by being heated with the laser beam of the first intensity level, there is a means for causing a new magnetization in the previously demagnetized portion of the second magnetic layer by applying an external bias magnetic field so as to have a new magnetization, the polarity of which is opposite to the predetermined polarity, and for causing the new opposite polarity magnetization in the second magnetic layer being transferred to the previously demagnetized portion of the first magnetic layer by magnetic exchange interaction resulting a portion of the first magnetic layer being magnetized in opposite polarity to the predetermined polarity. The transferred opposite polarity magnetization represents "1" to "0" of binary code. In order to magnetize the portion of the first magnetic layer in a polarity same as the predetermined polarity to represent "0" or "1", the storage medium already initialized is irradiated with the laser beam of the second intensity level so that the only the portion of the first magnetic layer is demagnetized, and when the portion is cooled down, the magnetization having the predetermined polarity at the corresponding portion in the second magnetic layer is transferred to the demagnetized portion of the first magnetic layer by the magnetic exchange interaction. There is also a means for reading out the data written in the first magnetic layer.
In the well known magneto-optical overwrite system (1), the high frequency magnetic field which changes at a high frequency must be generated for high density-writing, since it is required to generate a magnetic field by the data signals to be written.
It is, however, difficult to generate a magnetic field of sufficient strength at a high frequency for fast data transfer.
Furthermore, in the magneto-optical overwrite system (2) employing double-layered films of magnetic exchange interaction, high data transferring speed is realized as data signals are written by an optical modulation means. It is not, however, easy to compose the first and second magnetic layers forming the double-layered films having magnetic exchange interaction therebetween in such a manner that the Curie-temperatures, the compensation temperatures, the coercivity, the magnetizations, the thickness of the films and the wall energy between the layers are controlled so as to have required characteristics with productivity, and repeatability, etc. Furthermore, the two external magnetic field applying means, that is, the means for initializing the second magnetic layer and the means for applying a bias magnetic field for writing data are required. This results in the apparatus becoming bulky.
Then, in the overwrite method (2) employing the magneto-optical storage medium of the double-layered films having magnetic exchange interaction, it is explained hereafter with reference to FIGS. 1A, 1B and 1C how it is difficult to compose such double-layered films of the magneto-optical storage medium to have the required magnetic characteristics. In the first and second magnetic layers of the magneto-optical storage medium having the magnetic exchange interaction to be employed in the overwrite system, when the coercivity and the Curie-temperature of the first magnetic layer at the room temperature are Hc.sub.1 and Tc.sub.1, respectively and those of the second magnetic layer are Hc.sub.2 and Tc.sub.2, respectively, the first and second magnetic layers are composed of magnetic materials with the magnetic characteristics in which the following relationships must be established. EQU Hc.sub.1 &gt;Hc.sub.2 ( 1) EQU Tc.sub.1 &lt;Tc.sub.2 ( 2)
Due to the magnetic exchange interaction, the magnetic transition characteristic of the laminated two magnetic layers each of which is composed of amorphous alloy consisting of a heavy rare earth metal (RE) and a transition metal (TE), is different from that of each of the magnetic layers behaving individually.
As is understood by Japanese Jounal of Applied Physics Vol. 20, No. 11, November, 1981, pp. 2089-2095, when the magnetic hysteresis characteristic of a single magnetic layer i.e. the first magnetic layer alone composed of an amorphous alloy consisting of a heavy rare earth metal (RE) and a transition metal (TM) is such as shown in FIG. 1B and the magnetic hysteresis characteristic of the second magnetic layer alone composed of an amorphous alloy consisting of a heavy rare earth metal (RE) and a transition metal (TM) exists, is such as shown in FIG. 1A the magnetic hysteresis characteristic of the laminated layer made up of the first and second magnetic layers is such as shown in FIG. 1C.
This suggests that the range of magnetic transition caused by the first magnetic layer shown in FIG. 1C is smaller than Hc.sub.1 by the first magnetic layer alone shown in FIG. 1B and the range of magnetic transition caused by the second magnetic layer shown in FIG. 1C is larger than Hc.sub.2 by the second magnetic layer alone shown in FIG. 1A. The following equations (2) and (3) are then established. EQU H.sub.1 =Hc.sub.1 -.sigma..omega./2M.sub.s1 t.sub.1 ( 2) EQU H.sub.2 =Hc.sub.2 +.sigma..omega./2M.sub.s2 t.sub.2 ( 3)
(where t.sub.1 and t.sub.2 are the thicknesses of the respective magnetic layers and .sigma..omega. is the interface wall energy.)
In the overwrite system employing the magneto-optical storage medium having the double-layered films, the initializing magnetic field H.sub.ini should be satisfy the following equation (4) referring to the magnetic transition ranges H.sub.1 and H.sub.2 shown in the equations (2) and (3). EQU H.sub.1&gt; H.sub.ini&gt; H.sub.2 ( 4)
The magnetic transition ranges H.sub.1 and H.sub.2, however, are related to the thicknesses t.sub.1 and t.sub.2 of respective magnetic layers and the wall energy, etc. as indicated by the equations (2) and (3).
Accordingly, in order to obtain the magnetic transistion ranges H.sub.1 and H.sub.2 for satisfying the condition of the equation (4), it is required to accurately control the coercivities Hc.sub.1 and Hc.sub.2 of respective magnetic layers, the saturation intensities of magnetization Ms.sub.1 and Ms.sub.2, the thicknesses of the films t.sub.1 and t.sub.2, etc., but, it is extremely critical to do so.
In order to solve the problem described above, the assignee of this application has already proposed a magneto-optical overwrite system comprising a magneto-optical storage medium having a portion composed of first and the second magnetic layers, each of which is composed of an amorphous alloy consisting of a heavy rare-earth metal and a transition metal, an isolation layer interposed between the first and second magnetic layers for isolating magnetic exchange interaction between the layers, a coercivity H.sub.1 of the first magnetic layer and a coercivity H.sub.2 of the second magnetic layer having a relationship Hc.sub.1 &gt;Hc.sub.2 at a room temperature, a Curie-temperature Tc.sub.1 of the first magnetic layer being higher than the room temperature and a Curie-temperature Tc.sub.2 of the second magnetic layer being higher than the room temperature having a relationship Tc.sub.1 &lt;Tc.sub.2 and at least the second magnetic layer being composed of a magnetic material having a compensation temperature below the room temperature and a substrate for supporting the first and second magnetic layers and the isolation layer, means for magnetically initializing both the first and second magnetic layers, an electro-magnetic energy beam being modulated in intensity into a first level and a second level forming a spot in the magneto-optical storage medium, the first level causing spotted portions of the first and second magnetic layers temperatures to rise over the Curie-temperatures Tc.sub.1 and Tc.sub.2, respectively, so that both of the spotted portions are demagnetized, the second level causing a spotted portion of the first magnetic layer temperature to rise to a temperature between the Curie-temperatures Tc.sub.1 and Tc.sub.2 so that only the spotted portion of the first magnetic layer is demagnetized, and means for moving the magneto-optical storage medium with respect to the spot of the electro-magnetic energy beam.
The magneto-optical overwrite system already proposed by the assignee of this application therefore does not require a writing bias magnetic field essential in the conventional systems. Furthermore, the initializing magnetic field generating means can be provided anywhere in the system other than the portion irradiated by a laser beam for writing. However, the initializing magnetic field generating means is essential in the system already proposed by the assignee of this application. This results in the system still becoming bulky.